The neurotransmitter 5-hydroxytryptamine (5-HT, serotonin) modulates a wide range of physiological and pathological processes in the central nervous system and periphery, including anxiety, sleep regulation, aggression, feeding and depression (Hoyer et al., Pharmacol. Rev. 46, 157-204, 1994). Both pharmacological characterization and molecular cloning of several 5-HT receptor genes has revealed that 5-HT mediates its diverse physiological actions through a multiplicity of receptor subtypes. These receptors belong to at least two different protein superfamilies: ligand-gated ion channel receptor (5-HT3) and the G-protein-coupled 7-transmembrane receptors (thirteen distinct receptors cloned to date). In addition, within the G-protein-coupled receptors, serotonin exerts its actions through a multiplicity of signal transduction mechanisms.
The cloning and characterization of the human 5-HT5A serotonin receptor has been described in FEBS Letters, 355, 242-246 (1994). The sequence is not closely related to that of any previously known serotonin receptor, with the best homology being 35% to the human 5-HT1B receptor. It encodes a predicted 357 amino-acid protein, with seven putative transmembrane domains, consistent with that of a G-protein coupled receptor. The sequence is characterized by containing an intron between transmembrane domains V and VI. More recently coupling to Gi/o α mechanisms has been demonstrated with the inhibition of forskolin stimulated cAMP, and also evidence for more complicated G-protein mediated coupling mechanisms have been proposed (Francken et al. Eur. J. Pharmacol. 361, 299-309, 1998; Noda et al., J. Neurochem. 84, 222-232, 2003). Furthermore, in WO 2004/096771 it is described the use of compounds, which are active on the 5-HT5A serotonin receptor for the treatment of depression, anxiety disorders, schizophrenia, panic disorders, agoraphobia, social phobia, obsessive compulsive disorders, post-traumatic stress disorders, pain, memory disorders, dementia, disorders of eating behaviors, sexual dysfunction, sleep disorders, withdrawal from abuse of drugs, motor disorders such as Parkinson's disease, psychiatric disorders or gastrointestinal disorders. The Journal of Psychiatric Research, 38, 371-376 (2004) describes evidence for a potential significant role of the 5-HT5A gene in schizophrenia and more specifically in patients with later age at onset.